There is no such thing as "native Ubuntu" :-). The default widget style of a modern Ubuntu installation is just a GTK theme (which, I think, supports both GTK 3 and GTK 2 -- the two are incompatible, so themes need to implement support for both). You can configure Qt to use the GTK 2 rendering engine, but GTK 2 and GTK 3 look slightly different for reasons that I am entirely unable to convey without an inflammatory post, so Qt (and GTK 2) applications won't look exactly the same as GTK 3 applications.

I am not sure how this is going (this blog post is the only info I can find on it) but they have plans to make a new UI library for Qt that is backed by the platform's native widgets, like AppKit on macOS:

http://blog.qt.io/blog/2017/02/06/native-look-feel/

Ubuntu is gtk-based, so you need a matching Qt-theme for that to work, it will look native on kubuntu/kde-based distros.

Thanks! I think Qt never uses "native" widgets. If at all, it just renders them to look that way.

wxWidgets is the only framework I can think of that uses native widgets.

There is also libui: https://github.com/andlabs/libui

Cool! Though it says it's alpha software?